Mechanisms Determining Crispness and Its Retention in Foods with a Dry Crust - Food Colloids: Self-Assembly and Material Science - page 492

consequences. When eating a crispy product the sound emission usually lasts
for a few hundred milliseconds; moreover, several sound pulses are heard.
Humans can only hear single sound pulses as individual sound events when the
time interval between the pulses is at least 3-5 ms. 37,38 A short passage in piano
music is perceived as a single acoustic event when an artificial pause between
two parts is shorter than 10-15 ms. 39 For a minimum of three individual sound
events to be heard, this implies that the crack must run at high speed for at least
10-15 ms, which implies a minimum size of the fracturing food of 3-5 m. This
is, of course, unrealistic. So the conclusion is that, during the eating of a crispy
product, the fast growing cracks must be initiated several times at different
places, and they must stop after some time. So the presence of 'crack stoppers'
is essential for a product to be perceived as crispy.
Large pores in the cellular structure are likely to act as crack stoppers. The
minimum size of these crack stoppers must be such that the time interval
between two sound pulses is about 5 ms. During the biting of a product, the
cracks are initiated by the teeth penetrating the product at a speed of 30-40 mm
s 1 . The required minimum distance between two points of crack initiation is
then 150-200 mm. This is the combined size of the beam or lamella that is

fracturing and the pore size, resulting in a minimum pore size of

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100 mm.
Pores of this size are generally present abundantly in crispy foods (e.g., see
Figure 3). The thickness of crispy products varies roughly between 2 mm (e.g.,
the crust of bread, or a battered deep-fried snack) and 10 mm (e.g., roasted
bread), implying that crack initiation and stopping will occur several times
during biting, even if the crack propagation proceeds along a line perpendicular
to the outside.

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34.4.3 Need for Overlapping Sound Events
During the fracturing of a single beam, the amplitude of the single sound event
is relatively low. For instance, the measured sound pressure of the emitted
sound during the fracture of a single beam in toasted rusk roll has been found
to range from 0.5 Pa (or 80-88 dB) (Figure 1). 9 (Sound pressure in

decibels can be calculated from the sound pressure P in Pa using the relation:
1dB R 20 log P/20 10 6 ). The sound pressure emitted during biting, or cutting
at the speed of biting, of a toasted rusk roll is up to 100 dB. 9 These data show
that one needs some overlap of sound signals from individual fracture events in
order to reach the sound pressures normally observed during eating of these
crispy products.
On average the sound signal emitted during fracture of a single beam lasts for
at most 1 ms. 7,9 To produce a sound signal with 10-12 dB the sound pressure
should be increased by a factor of 3-4 times, and so there must be 3-4 fracture
events per millisecond - in fact, there must be rather more, since sound energy
is only high during about half a millisecond. The highest speed during biting at
which the teeth approach each other is 30-40 mm s 1 . Assuming that a product
that is eaten is on average 4 cm wide, we can calculate a maximum average size

0.2 to

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